This invention relates to a transparent or translucent multilayer film having at least one low gloss layer which may be readily bonded to paper. The film has particular utility for windows in envelopes.
It is known to use polymeric films for windows in paper envelopes. Such window envelopes save the resources necessary to separately address an envelope on its surface, or to apply an address label to the envelope. A letter, invoice, or other contents may be shaped to size, as by folding, as necessary, to place the address of the intended recipient appearing on the contents in a position to be visible through the window comprising the polymeric film.
Films having applicability as envelope windows are known. DE-A- 4312388.0 discloses a transparent stretched polypropylene film having a reflection-reduced surface, which is bondable. The film comprises polypropylene of at least two layers. Reduced reflection is obtained by incorporation of a substantially propylene/ethylene copolymer incorporating linear low density polyethylene and maleic anhydride. The disclosed films are stretched on two axes.
DE-A-4312388.0 reports that polystyrene film and PVC film have been used as window envelopes. Biaxially oriented polypropylene films have been used, but such films are there reported to be not readily bondable to paper by fast running envelope manufacturing equipment.
Polystyrene surfaces however are known to the manufacturers of envelopes. Adhesion products for bonding the window film to envelopes are known, and effective. Polypropylene however offers a cost advantage over polystyrene.
It is known to prepare films having layers of polypropylene and polyethylene from GB-A- 2281282, and GB-A- 2111908.
JP-A-51-28879 promotes a biaxially stretched film of propylene and laminate of styrene butadiene thermoplastic elastomer. This film is claimed to exhibit good heat-sealing characteristics.
JP-A-55-57428 discloses a film of polystyrene laminated on polypropylene which is subsequently biaxially oriented (stretched). Its use as tape suitable for wrapping of heavy objects is proposed.
JP-A-55-148155 discloses three-layer films comprising:
1) a layer of styrene and block polystyrene-butadiene in combination with a solvent for the styrene-butadiene block copolymer,
2) a flexible layer which may be polypropylene and ethylene-vinyl acetate co-polymer, and
3) a drawn layer which may be polypropylene.
The film is proposed as easy-open storage bags for food, which remains pliable at freezing temperatures, yet may withstand boiling water.
JP-A-55-163164 also discloses a food container formed from a film of biaxially stretched polystyrene onto which a polypropylene layer is cast or laminated.
JP-A-80-23746 discloses a food packaging film of polystyrene, having a polypropylene layer of 1/10 to 1/100 the thickness of the polystyrene layer. The layer film is stretched.
JP-A-56-27326 discloses a laminated sheet of polystyrene/butadiene/styrene and polypropylene which is biaxially stretched. The film is proposed for adhesives tapes.
JP-A-89-174324 discloses a biaxially stretched polypropylene film having reduced gloss and luster, as a wrapping material. The polypropylene is coated with an emulsion containing styrene, after corona discharge treatment.
JP-A-63-110150 teaches lamination of polystyrene film and a polyolefin film for use as bags.
NL-A-69-13608 discloses stencil film of oriented polypropylene film optionally laminated with polystyrene.
U.S. Pat. No. 4,608,284 teaches a laminate of an oriented polystyrene film, which may have been printed on the laminated surface, and a polypropylene film which has not been oriented. An adhesive joins the films at their interface surfaces. The films find use as heat-shrinkable printed labels.
U.S. Pat. No. 5,468,563 discloses films asserted to have utility as window film prepared from polypropylene. A reflection reduced layer is suggested of rubber modified ethylene and maleic anhydride, co- and terpolymer of propylene/ethylene/butylene having propylene as the principal component, or copolymers of propylene/polyethylene block copolymers. The core layer is comprised of isotactic polypropylene, a random propylene co-polymer. A second outer layer is constituted from the materials of the core layer having anti-block agents added. The layers may be co-extruded, then stretched
EP-A-01 22495 discloses films of biaxially oriented polypropylene, laminated to an olefin polymeric film having at least three peaks between 120xc2x0 C. and 65xc2x0 C. when analyzed by DSC (differential scanning calorimetry) . The multiple peaks are said to produce low gloss and a high haze. End use as a decorative wrapping paper having a metallized surface thereon for example by lamination is suggested.
JP-A-51-19079 discloses a multi-layer composite film in which one layer is formed from a polyolefin (such as polypropylene) and one layer is a thermoplastic layer obtained by adding 5-30 parts by weight of a styrene-butadiene copolymer to a polystyrene resin. The multi-layer film is manufactured by co-extrusion and thermal fusion of the layers. No orientation of the films is carried out.
U.S. Pat. No. 4,464,439 discloses a co-extruded laminate formed from at least one sheet of crystalline polypropylene and at least one sheet of polymeric material consisting of a mixture of high-impact polystyrene, crystalline polypropylene and styrene/dienic monomer block copolymer. No orientation of the films is carried out.
GB-A-1 357950 discloses a thermoplastic laminate having at least two layers, one of which is a ternary blend of a homopolymer or copolymer of a vinyl aromatic monomer; a homopolymer or copolymer of an olefin monomer; and a block copolymer having a polyvinyl/aromatic block and a polyolefin block. No orientation of the films is carried out.
Films for envelope windows require several properties. As noted in U.S. Pat. No. 5,468,563, several important properties include: bondability of the window film to the paper of the envelope using bonding agents conventional in the industry and transparency in order that the address is readable through the envelope by postal sorting machines. While transparency is important a window that is transparent and also glossy reflects light. The reflected light may impair the efficiency of the postal sorting machines and indirectly the arrival date of the envelope and contents to the addressee.
According to the invention there is provided an oriented polymeric film comprising a co-extrudate of a first layer having at least 96 weight percent of a polypropylene and of an external layer having (by weight)
a) from 70 to 97% of polystyrene, rubber modified polystyrene or a blend thereof, and
b) from 3 to 30% of a polypropylene/polystyrene compatibilizer selected from the group of ethylene vinyl acetate copolymer and block or random copolymers of a vinyl aromatic compound and an olefin, wherein the film has a minimum stretch ratio of 3 in the machine direction, and 4 in the transverse direction.
Multi-layered polymeric films may be formed by co-extruding the film layers. However, when a film is formed from a co-extrudate of two layers in accordance with the invention, it is found that the co-extrudate layers are separable simply by pulling them apart by hand, for example by using office adhesive tape adhered to each side of the film.
It has now been discovered that, if the co-extiudate layers are oriented in both the machine direction and the transverse direction after the film has been co-extruded, then this results in a film in which the co-extrudate layers are extremely difficult to separate.
Another advantage of the oriented films is that they have a higher modulus (higher rigidity) than non oriented films. Rigidity (high modulus) is a property required for window envelope film for two reasons: for the envelope converter to allow an optimum window patch unit feeding on the envelope machine, and for the final envelope to minimize wrinkling/buckling of the window panel.
The xe2x80x9cfirst layerxe2x80x9d comprising polypropylene is from time to time referred to herein as a xe2x80x9ccore layerxe2x80x9d, since for reasons of economy, it will usually be the predominant layer, and because frequently outer layers are provided on both sides of the polypropylene layer. However, the description xe2x80x9ccore layerxe2x80x9d should not be taken to imply that external layers are necessarily provided on both sides of the core layer.
The applicants have observed that polystyrene meets many of the limitations required by envelope window film. Polyolefins however provide cost advantage.
The films according to the invention are advantageously co-extruded of a comparatively less expensive core layer of polypropylene. A (usually thinner) layer comprising substantially polystyrene provides the required gloss and bondability to paper when used with conventional glue for window film applications.
The polypropylene employed in the core layer may be a homopolymer or copolymer and may preferably be predominantly isotactic polypropylene having a density of from 0.895 to 0.910, and a melt index of from 0.5 to 10 g/10 min (230xc2x0 C./2.16 kg.). Optionally, additional components conventional to the film industry may be added to the polypropylene core layer for example, additives for control of slip, such as erucamide, stearamide, or oleamide; additives for control of block such as silica, or calcium carbonate; additives for control of static such as ethoxylated fatty acid amides, alkyl benzene sulfonates, or polyethylene glycol esters; additives to aid clarity of the film such as dibenzylidene sorbitol; and additives to aid processing such as a fluoroelastomer, or a polyamide. The amounts for each optional component may range from zero to one percent by weight. In the aggregate all optional components generally do not exceed four, preferably not more than three per cent by weight of the polypropylene layer.
In one embodiment, the amount of polystyrene is from 50% to 95% by weight and the amount of rubber modified polystyrene is from 0 to 50% by weight based on the total weight of the external layer.
The polystyrene employed in the external or low-gloss/bonding layer is preferably general purpose polystyrene (GPPS), preferably GPPS having a melt index of from 0.5 to 15 g/10 min (200xc2x0 C. 5 kg weight).
Optionally present in the external or low-gloss/bondable layer is high impact polystyrene (HIPS). The HIPS may be present in an amount of from zero to fifty per cent by weight of the low-gloss layer. The HIPS preferably has a melt index of from 0.5 to 15 g/10 min (200xc2x0 C. 5 kg weight) and a rubber content of from 4 to 15 weight percent.
The external or low-gloss/bondable layer also includes from three to thirty percent, arid preferably 15%, of a compatibilizing compound useful to compatibilize polypropylene and polystyrene. A preferred compatibilizing component is an ethylene vinyl acetate copolymer having a vinyl acetate content of from fifteen to thirty weight percent and a melt index of from 0.5 to 25 g/10 min (190xc2x0 C. 2.16 kg weight). A further preferred compatibilizer is a block or random copolymer of a vinyl aromatic compound and an olefin. Suitable compatiblizing components which are styrenic copolymers include: styrene-butadiene-styrene (SBS); styrene-isoprene-styrene (SIS); styrene-isoprene (SI); styrene-butadiene (SB); styrene-ethylene-butylene-styrene (SEBS) having a melt index of from 0.5 to 35 g/10 min (200xc2x0 C. ,5 kg weight) and a styrene content of from 10 to 50 weight percent; and ethylene-styrene interpolymers having a melt index of from 0.5 to 40 g/10 min (190xc2x0 C. 2.16 kg weight) and a styrene content of from 5 to 80 weight percent.
The presence of the compatibilizing component in the external layer is also an advantage for the disposal of film which is not of a grade suitable for sale. Small amounts of the co-extruded film including the low-gloss bonding layer which comprises polystyrene film may be included with polypropylene core layer as a xe2x80x9cre-grindxe2x80x9d. In this manner waste generated in manufacture may converted to useful products. Incorporation of re-grind film is advantageously at a minimum, but up to 25 per cent by weight of re-grind may be incorporated in the propylene core layer when compatibilizers within the ranges described are included in the external layer.
Excess gloss results in light reflection from the film surface. Reduction of gloss generally also leads to opacity of the film, thus making the address difficult to read through the film. The gloss level of the film is preferably below 105%, more preferably below 100%.
The film is preferably extruded from a slot or a circular die.
The orienting of the film occurs after the film is co-extruded. The film may be re-heated to from 120xc2x0 C. to 170xc2x0 C., and oriented in the Machine Direction (MD) to form a film from 3 to 7, preferably from 4 to 6, times the original length and in the transverse direction of from 4 to 12, preferably from 5 to 9, alternatively from 6 to 10 times the original width. The orientation of the film may occur simultaneously or sequentially. Different re-heating temperatures may be used for the MD and TD orientations. The oriented film may optionally then be annealed for example at a temperature of from 5 to 15xc2x0 C. above the orientation temperature. Each step in the process of orienting the film is generally completed in a few seconds, for example less than three seconds, after the film reaches the chosen temperature.
The co-extrudate consists of 2 or more layers. Additional low-gloss/bonding layers can improve the low gloss properties even though the additional low gloss layer may be on the opposite side of the film from the measured surface.
It is important for window film that the film is able to form a strong bond to paper. The bond strength is preferably at least 3 N/2.5 cm, preferably at least 4 N/2.5 cm.
In a second aspect of the invention, there is provided a polymeric film comprising a co-extrudate of a first layer having at least 96 weight percent of a polypropylene and of an external layer having (by weight)
a) from 50% to 95% polystyrene
b) from 0 to 50% rubber modified polystyrene
c) from 0 to 30% of a polypropylene/polystyrene compatibilizer, selected from the group of ethylene vinyl acetate copolymer and block or random copolymers of a vinyl aromatic compound and an olefin, wherein the sum of components b and c is from 5% to 50% of the external layer. In an alternative embodiment, said film is orientated as described above, and in the preferred ratios described above.
In a third aspect of the invention, there is provided a window envelope having a transparent panel, which panel is formed from a polymeric film as described hereinbefore.
In a fourth aspect of the invention, there is provided the use of a polymeric film as described hereinbefore as a transparent panel in a window envelope.